The annual world market for diagnostic equipment based on immunoassays has increased considerably in the last few decades. The main reason for the success of immunoassays is that the method is general and easy to adjust to various chemical analysis problems. By using different types of detection techniques in combination with immunoassays, a number of important chemical substances can be identified and quantified. Depending on the physical measuring principle, different types of detectors are suitable for different types of analysis problems. Since the introduction of immunoassays, a great deal of detectors with excellent performance have been presented. One type of detector uses magnetic permeability as the basis for detection. Such a detector, which is described in SE 9502902-1 and U.S. Pat. No. 6,110,660, allows quick and simple identification of substances using immunoassay technology. The measurements are carried out by placing samples in a measuring coil whose inductance is measured and compared with a separate air-filled reference coil. This type of device allows measuring of magnetic permeability of samples, but it suffers from the drawback that the temperature-dependent drift limits the sensitivity of the detector. The temperature drift is caused by variations in the temperature of the sample and by the fact that the temperature of, respectively, the measuring coil and the reference coil is affected differently by the actual measuring process.
The present invention solves the problem of temperature-dependent drift in a new and efficient manner when measuring magnetic permeability or, alternatively, relative magnetic permeability. Furthermore, it makes it possible to obtain other parameters from the collected measuring data, which parameters are connected to magnetic permeability, for example magnetic susceptibility.
Magnetic immunoassays are based on the principle that a sample is introduced into a sample container, containing one or more magnetic reagents and a liquid, and then the sample container is placed in an instrument for reading the concentration of an analyte. (Kriz et al., Analytical Chemistry 68, p 1966 (1996); Kriz et al., Biosensors & Bioelectronics 13, p 817 (1998); Larsson K. et al., Analysis 27, p 78, 1999).
The above-mentioned documents, SE 9502902-1, U.S. Pat. No. 6,110,660 and Larsson K. et al., Analysis 27, p 78, 1999, disclose prior-art devices and methods, in which use is made of detection of magnetic permeability for quantitative chemical analyses of samples placed in a measuring coil. Said devices and methods do not, however, comprise an integrated double coil, i.e. a measuring coil and a reference coil which simultaneously surround a sample container. Consequently, there is no continuous temperature drift compensation, which means that the temperature of the sample has to be kept constant. It is difficult in practice, and in some cases even impossible, to control the temperature of the sample during the measuring process, in particular when it is placed in the measuring coil during the actual measuring process.
Other prior-art techniques also comprise a flow detector for liquid chromatography based on measurements of Nuclear Magnetic Resonance, NMR (Spraul M. et al., NMR Biomed 7, 295-303, 1994). However, this detector does not measure the magnetic permeability which, unlike NMR, is a macroscopic property originating from the outside of the atomic nucleus in a material. In addition, this device does not comprise a double coil as in the present invention.